A technique for forming a thin film transistor (TFT) by using a thin semiconductor film formed over a substrate having an insulating surface has attracted attention. A thin film transistor is used for a display device typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a thin semiconductor film that can be applied to a thin film transistor, and in addition, an oxide semiconductor has attracted attention as another material that can be applied to a thin film transistor.
As a material of the oxide semiconductor, zinc oxide or a material containing zinc oxide as its component is known. Further, a thin film transistor formed using an amorphous oxide (an oxide semiconductor) having an electron carrier density of lower than 1018 cm−3 is disclosed (Patent Documents 1 to 3).